In the topsy-turvy world of the US Congress nothing is certain until the votes are counted.

But today's announcement that the Democratic and Republican leadership will support President Barack Obama's plans for strikes in Syria all but guarantees that American cruise missiles will soon be heading for Damascus.

Standing outside the White House after a meeting in the Cabinet room, John Boehner, the Republican speaker of the House, said the words that anxious administration officials had been waiting to hear.

"I'm going to support the President's call for action," Mr Boehner said. "I believe that my colleagues should support this call for action."

Almost as important was the backing of Eric Cantor, the House's number two Republican, who in the past has scuppered his party's rare deals with the White House.

With the two men on side, as well as the Democrat leader Nancy Pelosi, Mr Obama now has key allies as he takes his plan for war into the lion's den of the Republican-controlled House of Representatives.

"Having Speaker Boehner, Leader Cantor and Leader Pelosi all supporting military action, an authorisation for some limited use of force is all but certain to pass on a bipartisan basis," said one Senate aide.

Why is it still too early to say for sure? Because although the White House now has the support of both party's leaders, those leaders don't have the full support of their members.

On the Republican side there are there is an increasingly assertive band of libertarians, opposed to foreign entanglements in the Middle East and determined to rein in the war powers of the post-9/11 presidency.

Their mirror image on the Left are the liberal Democrats who are unafraid to oppose to military force even when their own man controls the Oval Office.

But while both groups will be widely heard on the cable networks they are unlikely to be able to stop America's march to war.

An instructive example is a bill proposed last month by Representative Justin Amash, a Republican from Michigan, which would have severely curbed the spy powers of the all-seeing National Security Agency (NSA).

Mr Amash's amendment was supported by the libertarian Right and the progressive Left and opposed by both party's leadership and the White House - a formula likely to be repeated when Congress votes next week on action in Syria.

Despite some anxious moments, the establishment narrowly prevailed over the rag-tag coalition in a 217-205 vote, and the power of America's military and intelligence agencies was re-affirmed. The expectation is the Syria vote will go on similar lines.

When Mr Obama puts his seat back on Air Force One en route to Sweden tonight he may not sleep easy - especially with the prospect of two days of argument over Syria with Russia's president, Vladimir Putin, at the G20 meeting later in the week. But at least he can briefly close his eyes knowing that the votes for war seem to be all but in the bag.